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Introduction to Functions Lab

Learning Goals

  • Practice reading tests and test errors
  • Practice writing functions
  • Learn how to use JavaScript's toUpperCase() and toLowerCase() methods
  • Practice using return and console.log()
  • Practice creating conditional statements

Introduction

Welcome to the JavaScript functions lab! You'll notice a few new things in this lesson that we haven't encountered before. Don't worry, we'll walk you through them.

If you haven't already, fork and clone this lab into your local environment. Remember to fork a copy into your GitHub account first, then clone from that copy. Navigate into its directory in the terminal, then run code . to open the files in Visual Studio Code. (If you are using a different code editor, the command will be different.)

Code-Along

To start, open up index.js in your text editor. You should see, well, nothing. We'll fix that soon.

Now open up test/root.js. Hey, there's something! What's all of this stuff doing?

At the very top of the file, you'll see

global.expect = require("expect");

const babel = require("babel-core");
const jsdom = require("jsdom");
const path = require("path");

This might be a bit bewildering, but all we're doing is referencing different libraries that help us run the tests. You may recall that we talked about packages in an earlier lesson. A library is similar: it's code that someone else (usually multiple someones) wrote for our use. The code above makes the libraries available inside our test environment, which enables us to run the tests.

If you go to test/indexTest.js, you'll see

describe("shout(string)", function () {
  // there's stuff in here, too
});

describe is a function provided by our test runner (in this case, we're using Mocha) — it's basically a container for our tests.

Let's take a closer look at that describe():

describe("shout(string)", function () {
  it("receives one argument and returns it in all caps", function () {
    // we'll get to this in a sec
  });
});

These internal describe() calls are used for describing the functions that you're going to write. In this case, the test is saying, "Okay, I think there's going to be a function called shout, and it should take one argument (it doesn't actually matter what the argument is called, but string, is nice and specific, don't you think?). It should return that argument in all caps.

Finally, we have

expect(shout("hello")).toEqual("HELLO");

which says that it expects a call to shout() with the string 'hello' will equal the string 'HELLO'. This is the actual test — otherwise called a spec, expectation, or assertion — for this function. We can have more than one test per function, but let's start with this one.

Running the Tests

To run the tests, first run npm install in the terminal to install the dependencies, then run npm test. The first output you'll see will look something like this:

$ npm test
> [email protected] test
> mocha --timeout 5000 -R mocha-multi --reporter-options spec=-,json=.results.json



  shout(string)
    1) receives one argument and returns it in all caps

  whisper(string)
    2) receives one argument and returns it in all lowercase

  logShout(string)
    3) takes a string argument and logs it in all caps using console.log()

  logWhisper(string)
    4) takes a string argument and logs it in all lowercase using console.log()

  sayHiToHeadphonedRoommate(string)
    5) returns "I can't hear you!" if `string` is lowercase
    6) returns "YES INDEED!" if `string` is uppercase
    7) returns "I would love to!" if `string` is "Let's have dinner together!"`


  0 passing (120ms)
  7 failing

  1) shout(string)
       receives one argument and returns it in all caps:
     ReferenceError: shout is not defined
      at Context.<anonymous> (test/indexTest.js:4:5)
      at processImmediate (internal/timers.js:461:21)

  2) whisper(string)
       receives one argument and returns it in all lowercase:
     ReferenceError: whisper is not defined
      at Context.<anonymous> (test/indexTest.js:10:5)
      at processImmediate (internal/timers.js:461:21)

  3) logShout(string)
       takes a string argument and logs it in all caps using console.log():
     ReferenceError: logShout is not defined
      at Context.<anonymous> (test/indexTest.js:18:5)
      at processImmediate (internal/timers.js:461:21)

  4) logWhisper(string)
       takes a string argument and logs it in all lowercase using console.log():
     ReferenceError: logWhisper is not defined
      at Context.<anonymous> (test/indexTest.js:30:5)
      at processImmediate (internal/timers.js:461:21)

  5) sayHiToHeadphonedRoommate(string)
       returns "I can't hear you!" if `string` is lowercase:
     ReferenceError: sayHiToHeadphonedRoommate is not defined
      at Context.<anonymous> (test/indexTest.js:40:5)
      at processImmediate (internal/timers.js:461:21)

  6) sayHiToHeadphonedRoommate(string)
       returns "YES INDEED!" if `string` is uppercase:
     ReferenceError: sayHiToHeadphonedRoommate is not defined
      at Context.<anonymous> (test/indexTest.js:44:5)
      at processImmediate (internal/timers.js:461:21)

  7) sayHiToHeadphonedRoommate(string)
       returns "I would love to!" if `string` is "Let's have dinner together!"`:
     ReferenceError: sayHiToHeadphonedRoommate is not defined
      at Context.<anonymous> (test/indexTest.js:48:5)
      at processImmediate (internal/timers.j

Hm, seven failed tests - that seems like a lot to navigate. Let's go ahead and get Mocha set up to only show the first failing test. Find the test script in the package.json file and add the --bail tag to the end:

"test": "mocha --timeout 5000 -R mocha-multi --reporter-options spec=-,json=.results.json --bail"

Save the file and run the tests. You should now see something like this:

$ npm test

> [email protected] test
> mocha --timeout 5000 -R mocha-multi --reporter-options spec=-,json=.results.json --bail



  shout(string)
    1) receives one argument and returns it in all caps

  0 passing (159ms)
  1 failing

  1) shout(string)
       receives one argument and returns it in all caps:
     ReferenceError: shout is not defined
      at Context.<anonymous> (test/indexTest.js:4:5)
      at processImmediate (internal/timers.js:461:21)

Much better! Now let's see if we can get that first test to pass. Open up index.js.

When we write our code, we follow the guidance of the tests. Let's take a look at that first error:

  1) shout(string)
       receives one argument and returns it in all caps:
     ReferenceError: shout is not defined
      at Context.<anonymous> (test/indexTest.js:4:5)
      at processImmediate (internal/timers.js:461:21)

The output above comes from this test in indexTest.js:

describe("shout(string)", function () {
  it("receives one argument and returns it in all caps", function () {
    expect(shout("hello")).toEqual("HELLO");
  });
});

We know that the test is expecting a function called shout that accepts an argument. The error tells us that shout isn't defined. So let's start by declaring our function in index.js:

function shout(string) {
  // todo
}

We know we won't pass the test yet because our function doesn't do anything yet. However, if we rerun the test, it should now give us more information about what we need to do:

  1) shout(string)
       receives one argument and returns it in all caps:
     Error: Expected undefined to equal 'HELLO'
      at assert (node_modules/expect/lib/assert.js:29:9)
      at Expectation.toEqual (node_modules/expect/lib/Expectation.js:81:30)
      at Context.<anonymous> (test/indexTest.js:4:28)
      at processImmediate (internal/timers.js:461:21)

Anytime you see Error: Expected undefined to equal [something], that means that the test is looking for a return value. We can also see the description of what the function should do: it receives one argument and returns it in all caps.

Okay, so with that information, we know that our function should return whatever string is passed in as an argument:

function shout(string) {
  return string;
}

But we're still missing one piece: how do we make string all caps? JavaScript has a built-in method for that! It's called toUpperCase(). We can call it on any string:

"Hello!".toUpperCase(); // 'HELLO!'

Above, we've called it directly on a literal string but, of course, we can also call it on a variable. So let's try it with our shout() function:

function shout(string) {
  return string.toUpperCase();
}

and run our tests again. We get:

  shout(string)
    ✓ receives one argument and returns it in all caps

  whisper(string)
    1) receives one argument and returns it in all lowercase

  1 passing (147ms)
  1 failing

  1) whisper(string)
       receives one argument and returns it in all lowercase:
     ReferenceError: whisper is not defined
      at Context.<anonymous> (test/indexTest.js:10:5)
      at processImmediate (internal/timers.js:461:21)

Hey! We got one to pass! Time to tackle the next error!

Your Turn

Now it's your turn to get the rest of the tests to pass. Note that some of them require you to use console.log() instead of return — follow the guidance of the tests!

Instructions

In this lab, we're writing functions that "speak" at different volumes — they whisper or they shout. The next test is similar to the first:

1) whisper(string)
       receives one argument and returns it in all lowercase:
     ReferenceError: whisper is not defined
      at Context.<anonymous> (test/indexTest.js:10:5)
      at processImmediate (internal/timers.js:456:21)

This test is telling us that whisper(string) receives one argument and returns it in all lowercase. At the moment, the test is failing because whisper is not defined.

Note: Just like .toUpperCase() changes any string to all uppercase in JavaScript, .toLowerCase() (e.g., 'HELLO'.toLowerCase()) changes any string to all lowercase.

After you get whisper(string) passing, the next two tests will be checking to see if a specific string is logged when a function is called. You will still need to use the .toUpperCase() and .toLowerCase() methods for logShout(string) and logWhisper(string). Keep in mind, though, that these tests are not looking for return values, only logs.

The final function you need to create is sayHiToHeadphonedRoommate(). Your roommate is very into their music, so whispering can be a bit difficult, but they'll always hear you if you say, "Let's have dinner together!" This time, you will need to return different strings depending on the string passed into the function.

Note: Although there are 3 tests for sayHiToHeadphonedRoommate(), you only need to write one function. This function should be able to handle all three test conditions:

  • If the string that is passed into the function is all lowercase, the function should return "I can't hear you!"
  • If the string that is passed into the function is all uppercase, the function should return "YES INDEED!"
  • If the string that is passed into the function is equal to "Let's have dinner together!", the function should return "I would love to!"

How do we check if a string is all lowercase or all uppercase? Let's take a look using a REPL. Click on "Show files" and then the "index.js" file to see the code:

<iframe height="400px" width="100%" src="https://replit.com/@lizbur10/InsubstantialTanInternet?lite=true" scrolling="no" frameborder="no" allowtransparency="true" allowfullscreen="true" sandbox="allow-forms allow-pointer-lock allow-popups allow-same-origin allow-scripts allow-modals"></iframe>

In the first line we're defining a string variable. In the next two lines we log the original variable, then the variable after calling toUpperCase() on it. Finally, we're checking to see whether those two values are equal. If you click the Run button, you'll see that the original string is mixed case and the upper cased version of it is (not surprisingly) uppercase. It should not come as a surprise, therefore, that comparing them with the equality operator (===) returns false. What do you think will happen if we initialize string to "HELLO!" instead? Play around with different values for string in replit and see what happens. Also try doing the same thing with toLowerCase() and make sure you understand what's happening.

Basically, if we compare a string to its uppercased (or lowercased) version and the two values are equal, then we know that the original string is uppercase (or lowercase). So now that we know how to make these comparisons, how can we use them to conditionally return different strings? If you need a refresher, go back and review the lesson on Selection with Conditionals.

Remember that punctuation is important! Humans might be able to understand that "Lets have dinner together" is close enough to "Let's have dinner together!" and means the same thing, but JavaScript will not consider these equal!

Reminder: As you're working through the tests, you can run your code by running node index.js in the terminal. If you want to do this, remember that you'll need to call the function inside index.js. If you are just running tests, however, you don't need to include a function call — the tests will handle that for you.

After you have all the tests passing, remember to commit and push your changes up to GitHub, then submit your work to Canvas using CodeGrade. If you need a reminder, go back to the Complete Your First Software Engineering Assignment lesson to review the process.